WHO air quality database: relevance, history and future developments

Abstract Air pollution is the second most important risk factor for noncommunicable diseases, but air quality monitoring is lacking in many low- and middle-income countries. The World Health Organization (WHO) recently released its 2022 updated air quality database status report. This report contains data from about 6743 human settlements, a sixfold increase from 1102 settlements in its first publication in 2011, which shows that air pollution is increasingly recognized as a health priority at global and national levels. However, progress varies across the world. More than 90% of the settlements in the database are in high- and middle-income countries and areas mainly in China, Europe, India and North America. The database is crucial for increasing awareness of air pollution, and for calculating global exposures and the corresponding burden of disease attributable to air pollution. This article describes the progress made and challenges in collecting air quality data. The database uses official data sources which can be difficult to access and assess, because air quality monitoring is done by different government bodies or uses varying monitoring methods. These air quality data can be used by the health sector to engage in discussions on monitoring air quality to protect public health, and facilitate multisectoral engagement of United Nations agencies to support countries to conform with the 2021 WHO air quality guidelines. Although air pollution levels in most countries are higher than those recommended in the guidelines, any action policy-makers take to reduce air pollution will help reduce the burden of air pollution on health.


Introduction
Air pollution is a recognized, global health risk factor and is associated with close to 7 million deaths every year. 1 The availability of air quality data is essential to assess the magnitude, distribution and trends of the effects of pollution on population health.These data are also important for modelling air quality, which in turn is critical for informing regional and global policies on air quality.
The World Health Organization (WHO) has played a key role in increasing awareness on the health risks of air pollution.WHO has been responsible for regular updates of the air quality guidelines since the 1980s, [2][3][4] the production of scientific reports on debated aspects of air pollution, 5 and the compilation and publication of routinely measured data in the WHO air quality database. 62][13] Each public release of the early versions of the WHO air quality database, or of the guidelines, has attracted media attention, putting air pollution and its harmful effects in the spotlight.
This paper presents an overview of close to 50 years of WHO's activities in gathering air quality data and their use in global assessments and efforts to reduce health risks caused by air pollution.

Evolution of air quality data
Efforts to compile a database of urban air pollution levels covering the entire globe date back to the 1970s, with the start of the WHO urban air quality monitoring pilot project in 1973.Subsequently, the project evolved into a component of the global environmental monitoring system air programme, which since 1975, WHO and the United Nations have operated as a component of the global environmental monitoring system. 14The programme's original objective was to strengthen the monitoring of urban air pollution to improve data comparability among different countries, and to assess air pollution levels and trends at a global level.While the global environmental monitoring system was eventually phased out in its original form, UNEP has recently re-branded it as global environmental monitoring system air, with a shift in its focus to move beyond data management systems to partnership development, awareness-raising, capacity development and knowledge exchange. 157][18] After 2015, the database, also referred to as the WHO air quality database, became an essential part of WHO's role of monitoring indicators 11.6.2(air quality in cities) and 3.9.1 (mortality attributed to air pollution) of the sustainable development goals (SDGs).
Since the first publication of the database in 2011, WHO has published four updates, in 2014, 2016, 2018 and 2022.Throughout the years, data coverage has improved sixfold: from 1102 human settlements (this can cover cities, towns and/or rural areas) 19 included in 2011, to 6743 in 2022 (Fig. 1).While data on particulate matter in 2011 were available for 91 countries, the latest version of the database includes information from 118 countries (Table 1).Moreover, whereas 52% (573/1102) of settlements reported PM 2.5 in 2011, this proportion reached 64% (3872/6050) in 2022, thus increasing the availability of relevant data for health risk and impact assessment.In the most recent database, WHO also included data on nitrogen dioxide (NO 2 ), with 5667 settlements in more than 80 countries reporting NO 2 concentrations.This expansion recognizes the importance of NO 2 as an important health risk in cities, as well as the differences in NO 2 distribution patterns compared with particulate matter, requiring customized policy interventions for effective air pollution reduction.
One of the early objectives of the WHO air quality database was to feed into efforts to derive national population exposures based on global air quality models.These models estimate air pollution levels as a spatial continuum (with resolution of about 10 × 10 km), covering the whole world. 20,21The use of information on pollutant emissions and their atmospheric transport and reactions, coupled with remote measurements of pollutant concentrations from satellites and calibrated with ground measurements by complex statistical models, has resulted in estimates that are well correlated with the available observations. 18More importantly, this approach also allows estimation of pollutant concentrations in places that lack air quality monitoring data, and encourages improvement in global availability of monitoring data.Recent methodological updates incorporate satellite data with PM 2.5 component fractions which can, for example, enhance our understanding of source contributions and guide national policies and sourcespecific interventions.
The publication of the database has attracted considerable media attention -and even controversy -in many different countries, resulting in an increased awareness of the negative effects of air pollution on human health.The database also provides WHO with an opportunity to engage actively with countries on air pollution, bringing health to the centre of the discussion on policies on air pollution abatement. 22,23hile current efforts in collecting air pollution data focus on all areas (for example, cities, towns and rural areas), initial efforts mostly concentrated on urban settings. 6This focus also helped to start the WHO-led BreatheLife campaign, now joined by a network of 79 cities, countries and regions, which aims to bring together actors -such as, agencies in charge of air quality management, local government and civil society groups -to share knowledge and best practices in both monitoring air pollution and curbing its levels. 24he coverage of ground measurements of PM 2.5 compiled in the WHO database is not homogeneous around the world, and is concentrated in high-and middle-income countries and areas mainly in China, Europe, India and North America (Fig. 2).In the 2022 version of the database,  with more ground monitors in high-and middle-income countries. 6oteworthy is the fact that in densely populated areas where measurements are still sparse, the uncertainty in the population exposure estimates is much greater and so is the calculation of the burden of disease attributable to air pollution. 18,20Overall, the available data show that air pollution levels are high; with PM 2.5 and PM 10 concentrations in 94% (6338/6743) of the settlements not complying with WHO air quality guidelines.

Challenges
Since the first efforts to compile an air quality database, the primary and preferred sources of data have been official national and subnational reports and websites.When official data have not been available, values from research networks and/or peer-reviewed journals have been used in 13 countries.
Retrieving official data can be challenging when the responsibility of air pollution monitoring is held at the subnational level or is distributed between various national actors, making the data retrieval process less effective and more time-consuming.For example, data collection in some countries is not centralized but is rather the responsibility of cities, districts or states (for example, Australia, Brazil and Chile), and is conducted without national oversight and coordinated reporting.Another challenge is the collection of routine ambient air pollution measurements, which is the responsibility of multiple ministries such as the environment ministry, national statistical office and health ministry (for example, Qatar).
An additional challenge in compiling the WHO air quality database is the diversity of methods used to monitor and report air quality.Reasons for this heterogeneity include: the cost associated with procuring and maintaining reference grade monitors; 25,26 lack of or differences in regulations and technical requirements to determine the acceptability of measurements; and insufficient training and maintenance of qualified technical personnel.Given the pressure to report the status of air quality, for some countries, the use of low-cost sensors is a more attractive option.These sensors have an upfront cost of a few thousand United States dollars while reference grade monitors can have a capital cost of tens of thousands of United States dollars.However, the operating cost for maintenance, software management, analysis, quality control and quality assurance of the data from low-cost sensors is not negligible and should be carefully considered, as well as data ownership. 25,27s regards the database, its objective is to compile existing and publicly available annual mean concentrations of PM 2.5 , PM 10 and NO 2 from officially operated monitoring networks.However, the data collected in the database are not homogeneous as each country may use different instruments, measurement techniques, station siting protocols and classifications.They may also apply different data reporting and aggregation methods, time coverage, and standards for data quality control and validation.Data may be missed because of language, or may not comply with inclusion criteria due to missing metadata.Because of these limitations, any direct comparisons of particulate matter and NO 2 concentrations across countries and years must be done with caution. 6,28

Importance of air quality data
The increase in data availability and the collection of historical data on air pollution is important to better understand the status of air quality and monitor trends, including on emerging sources of air pollution such as wildfires and sand and dust storms, and progress in policies to reduce pollution.For example, elevated PM 2.5 concentrations in Australia in 2019 relative to previous years highlights the impact of extensive wildfires on air quality.On the other hand, decreases in PM 2.5 concentrations in China after 2013 illustrate the effect of national programmes to reduce air pollution.
The regular compilation and publishing of global air pollution data can also reduce restrictions on data sharing, and facilitate access to data collected by monitoring networks, by demonstrating the usefulness of a comprehensive assessment of air quality based on international data.The availability of air pollution data can stimulate health impact analysis and policy tracking activities that would otherwise be difficult. 29Access to these data can also support research in the field of exposure modelling and epidemiology.Although the data are not updated annually, during each update (every 2 to 3 years), capacity-building that aims to enhance the data collection process is undertaken through the training of government officials of WHO Member States via country consultation, discussions and webinars with exposure scientists (for example, atmospheric and modelling scientists) and epidemiologists.Capacity-building initiatives, such as local health impact assessment projects, are also shared by WHO Member States to promote knowledge translation.
Countries are sensitive about their air quality data, and it has taken time to build trust when monitoring the SDG indicators 11.6.2 and 3.9.1,especially to obtain input data.Yet, over the years, countries have engaged with WHO to share their data.Data ownership and understanding of the science underpinning the health impacts of air pollution are key, and supporting national institutions in monitoring air quality and conducting health impact assessments should focus on using their local data.This effort should be coupled with appropriate and sustainable capacity-building and resources.A good example of a science policy framework is the Convention on long range transboundary air pollution.The cooperative programme for monitoring and evaluation of the long-range transmission of air pollutants in Europe has shown the importance and value of using data validated by countries in a clearly defined and internationally established framework and methods. 30ver the past few years, methods to assess air quality have improved (for example, extensive use of satellites, lowcost sensors and modelling), allowing better coverage and space-time resolution of data. 18,27,31,32While countries may be tempted to change to these alternatives instead of maintaining reference grade monitors which are the gold standard, albeit costly, these new methods (whether measured or modelled) have not all been properly validated, and the cost-benefits of the individual and combined methods need to be assessed.As well as measuring air quality, it is impor-Bull World Health Organ 2023;101:800-807| doi: http://dx.doi.org/10.2471/BLT.23

Going forward
A unique opportunity has arisen to encourage and strengthen health-sector engagement in air quality, as countries are considering revising their national ambient air quality standards to better align with the 2021 update of the WHO air quality guidelines to protect public health.Health considerations need to be better integrated in policies of those sectors that are substantial emitters of air pollutants.4][35] It is important to recognize that the WHO air quality guidelines do not specify end goals that define success or failure, but rather they offer guidance of what better health can look like with cleaner air.In a perfect world, there would be no air pollution, but in the absence of that ideal, policy-makers must acknowledge that any action they take to reduce air pollution is a step in the right direction for public health.Advances in exposure assessments -for example, low-and middle-income countries providing exposure-response functions for WHO's 2021 air quality guidelines -reflect methodological improvements and progress in the scientific assessment of the health effects of air pollution.Local data compiled for regulatory purposes have been, and continue to be, regularly used to assess population exposure to ambient air pollution; monitor the effectiveness of policies and actions; assess health impact; derive burden of disease estimates; and undertake epidemiological research. 29et, data availability and quality are often not ideal, and fail to capture human exposure and measure exposure inequalities.For example, the number of monitors available to map population exposure tends to vary greatly across the world: 2-3 monitors per million persons in Europe; 0.5 monitors per million persons in Japan. 36In Africa, data are available from 41 cities in 11 of 47 countries, which means that no data monitoring exists for tens of millions of people.
Despite increased advocacy, the increase in monitoring of air quality has not been equal across the regions.The health community therefore needs to increase engagement in monitoring air quality to facilitate health-risk assessments, especially in view of the growing availability of new methods and technologies, for example, low-cost sensors and modelling or a combination.While low-cost sensors cannot replace reference grade monitors for long-term health impact assessments, several platforms have combined data from low-cost sensors with data from official reference grade monitors to provide global overviews of real-time air pollution that can raise awareness and inform public action. 37,38o strengthen engagement of the health sector to support national health impact assessments, WHO established the global air pollution and health technical advisory group to advise on methodological improvements for measuring air pollution exposure and its health effects. 39The group has provided guidance on several reports that have documented advances in methods to assess exposure to air pollution, 40 and epidemiological studies, 41 which can help countries increase local monitoring and health impact assessments.In the near future, the advisory group will support actions to respond to emerging threats, such as wildfires, and sand and dust storms, and guide the development of a global database on air pollution policies.
Currently, WHO is collaborating with academic institutions, and governmental and nongovernmental agencies across the world to develop training modules that cover exposure and health impact assessments among others topics, which can be conducted in person or via the OpenWHO platform. 42These training modules will also be adapted into webinars, which will help build the capacity of the health workforce to engage in multisectoral action with the transport and energy sectors to ensure health is considered in policy planning. 43HO has been working closely with the World Meteorological Organi-zation and UNEP over the years on air quality issues at the global level, both on advocacy and global modelling. 23,24HO and UNEP have a longstanding collaboration at the regional level in ministerial processes in countries in the Americas, South-East Asia and Western Pacific Regions, which has ensured harmonization of the communication strategy, and resolutions endorsed by ministries of the environment that support WHO's work on air quality and health.In the European Region, WHO collaborates with the United Nations Convention on Long-Range Transboundary Air Pollution as the secretariat of its joint task force on health.44 This collaboration was further strengthened by the UN inter-agency working group on reporting on SDG 11.6.2, and through the compilation of tools and guidance documents covering standards, measurements, modelling, and health and economic impact assessments, among others, that can support countries in working towards the WHO air quality guidelines.45 Collective harmonization is needed of national ambient air quality standards, definitions for monitoring air quality, and standardized methods and techniques, so that data collection and monitoring of progress can be efficiently conducted and can be used to inform decision-makers on what is working best to protect public health from air pollution.
To conclude, with almost 50 years of experience compiling air quality data, WHO's air quality database is important to demonstrate the magnitude and global distribution of health risks due to air pollution.The database can also help Member States to identify tools that can enhance their capacity to assess and reduce air pollution health risks, and actions to empower multiple sectors to advocate for clean air for healthier populations.■ Bull World Health Organ 2023;101:800-807| doi: http://dx.doi.org/10.2471/BLT.23

Resumen
Base de datos de la OMS sobre calidad del aire: relevancia, historia y desarrollos futuros La contaminación del aire es el segundo factor de riesgo más importante de las enfermedades no transmisibles, pero en muchos países de ingresos bajos y medios no se vigila la calidad del aire.La Organización Mundial de la Salud (OMS) publicó hace poco su informe actualizado de 2022 sobre el estado de la base de datos de calidad del aire.Este informe contiene datos de unos 6743 asentamientos humanos, es decir, seis veces más que los 1102 asentamientos de su primera publicación en 2011, lo que demuestra que la contaminación del aire se reconoce cada vez más como una prioridad sanitaria a nivel mundial y nacional.Sin embargo, los progresos varían en todo el mundo.Más del 90% de los asentamientos de la base de datos se encuentran en países y regiones de ingresos altos y medios, principalmente en China, Europa, India y Norteamérica.La base de datos es esencial para aumentar la concienciación sobre la contaminación del aire y para calcular las exposiciones globales y la correspondiente carga de morbilidad atribuible a la contaminación del aire.Este artículo describe los progresos realizados y los desafíos que plantea la recopilación de datos sobre la calidad del aire.La base de datos utiliza fuentes de datos oficiales a las que puede resultar difícil acceder y evaluar porque el control de la calidad del aire lo realizan diferentes organismos gubernamentales o utilizan métodos de control que varían.El sector sanitario puede utilizar estos datos sobre la calidad del aire para participar en debates sobre la vigilancia de la calidad del aire con el fin de proteger la salud pública y facilitar el compromiso multisectorial de los organismos de las Naciones Unidas para ayudar a los países a cumplir las directrices de la OMS 2021 sobre la calidad del aire.Aunque los niveles de contaminación del aire en la mayoría de los países son superiores a los recomendados en las directrices, cualquier medida que adopten los responsables de formular políticas para reducir la contaminación del aire contribuirá a reducir la carga de la contaminación del aire sobre la salud.

Fig. 1 .
Fig. 1.Human settlements with air quality data included in the WHO air quality database, by year of release